北磺溪位於台灣北部大屯火山群中,為其主要的流域系統,且大屯火山雖然已無火山活動,但因後火山作用旺盛,溫泉以及含硫噴氣口林立,當溪水與溫泉水混合之後,導致北磺溪溪水溫度偏高,酸鹼值偏低(3.0~4.5),且硫酸根離子濃度極高(40~600ppm)。這些特性促進了水與安山岩間的地質化學反應,使得北磺溪溪水高度富含了鐵、鋁、矽元素。這些元素在水中傾向於形成懸浮狀的膠體顆粒,並且吸附水中的重金屬元素。在氧化條件下,一旦水中酸鹼值提高(天水或地下水補注),這些懸浮狀的膠體顆粒會以一種赭石色的粉末沉澱在河床中。 本研究沿北磺溪上游至下游共採集5組的赭石色的粉末樣本,由X射線螢光分析及X射線繞射分析顯示,其主要成份的平均含量依序為Fe2O3(54.3%)、SiO2(15.2%) 、Al2O3(8.1%),其餘的主要元素成份含量大約在1%或以下,並且無明顯結晶構造,屬於非結晶性的水合物,可以定義此赭色的粉末為鐵-鋁氧化水合物。根據前人研究,此類鐵、鋁氧化物對水中的重金屬離子以及有機物是一種具有高潛力的吸附劑,其吸附作用取決於氧化物顆粒表面特徵及水體的酸鹼值。透過消化萃取顯示,包含砷、鉻、銅、鉛、鎳、鋅等重金屬元素,濃度皆在數百ppm以上。 為了評估重金屬元素與鐵-鋁氧化水合物的鍵結方式,本研究利用BCR連續萃取以及長時間連續萃取兩種方式,針對可交換態、可還原態以及可氧化態三種鍵結相態,將重金屬元素依照不同相態作一系列的分離萃取。一般而言,重金屬元素主要是與氧化物鍵結存在於可還原態中,並伴隨著鐵與鋁的還原反應一同溶解釋放。綜合兩種連續萃取方式其結果顯示,可還原態有最高的鐵以及鋁含量,但萃取出的重金屬含量相對的低,而大多數重金屬元素卻存在於可交換態以及可氧化態中,以離子鍵行表面吸附或是以錯合型式與硫化物結合,與人工製備的鐵、鋁氧化物有很大的差別。 另外為了評估天然或人為因素造成的環境改變,對重金屬脫附行為所造成之影響,本研究設計了酸鹼值脫附實驗、界面活性劑脫附實驗以及腐質酸脫附實驗三種方式對鐵-鋁氧化水合物進行1個星期的脫附實驗,顯示在鹼性環境下重金屬元素較不易脫附,而腐質酸溶液在與鐵-鋁氧化水合物對重金屬元素進行競爭吸附過程中,會搶走多數的重金屬元素並發生沉澱。整體來說,脫附實驗中被脫附的重金屬元素濃度介於數十ppb至數千ppb之間。
Tatun Volcano Group (TVG) is not active since late Pleistocene but the post-volcanic activities, such as hot spring and sulfur gas, still widespread around the volcanic area. Peihuang Creek is the main watershed system in TVG. The creek water is characterized by higher temperature, low pH values (3.0-4.5) and high SO4 content (60-400 ppm) due to the mixing of hotspring. This would promote the geochemical interaction between water and andesitic rocks and results in waters with highly enriched iron, aluminum and silica. These elements prefer to form suspended colloidal particles in water and adsorb heavy metals. Once the pH of water increases under oxidation condition, the colloid would precipitate in the form of ochre colored powder on the riverbed. Five samples of ochre colored powder were sampled along the creek. The results of XRF demonstrate that the powder is mainly composed of iron(54.3%), silica(15.2%) and aluminum(8.1%). No clear crystal structure can be determined by XRD; therefore, the ochre colored powder is defined as amorphous Fe-Al hydroxide. According to previous study, this kind of iron, aluminum oxides can adsorb a large amount of heavy metals. Using digestion method, that the amorphous Fe-Al hydroxides do adsorb houndreds to thousands of ppm heavy metals. To evaluate the binding phase of heavy metals, the BCR and long-term sequential extraction procedure were conducted. Generally, it is believe that heavy metals are mainly bonded with reducible phase. In this study, on the contrary, the results of sequential extraction show most heavy metals were released from exchangeable and oxidizable phases. In addition, more extraction procedures were conducted for simulating variable natural and anthropogenic environments, including pH-dependent extraction, surfactant extraction and humic acid extraction. The results show that heavy metals were hard to be extracted in high pH value because of the neutral pHzpc of Fe-Al hydroxides. The low solubility of humic acid also results in the removal of heavy metals from water. Conclusively, Heavy metals in tens to hundreds of ppb can be extracted under natural environments.